The present invention relates to body implantable tissue stimulation electrodes, e.g. for cardiac pacing or cardioversion/defibrillation, and more particularly to the deployment and implantation of such electrodes.
Heart disease is a major cause of deaths in the United States and in other industrialized nations. One well known treatment approach utilizes an implantable cardiac pacing device, through which relatively mild periodic electrical impulses are applied to epicardial or endocardial tissue as necessary to maintain normal sinus rhythm. More recently, cardioversion/defibrillation devices have been developed to counteract tachyarrhythmias (rapid disturbances in cardiac electrical activity). In particular, the conditions of ventricular tachycardia, ventricular flutter and ventricular fibrillation are widely believed to be the primary cause of sudden deaths associated with heart disease. Defibrillation devices also are utilized to counteract atrial tachyarrhythmic conditions, although such conditions are not considered life threatening unless they lead to a rapid ventricular disturbance.
Tachyarrhythmic conditions frequently can be corrected by applying relatively high energy electrical shocks to the heart, a technique often referred to as cardioversion. Cardioversion devices include implantable electronic standby defibrillators which, in response to the detection of an abnormally rapid cardiac rhythm, discharge sufficient energy through electrodes connected to the heart to depolarize and restore the heart to normal cardiac rhythm.
Cardioversion/defibrillation devices frequently include epicardially implanted electrodes. The surgical procedure required for implantation, i.e. thoracic surgery such as a median sternotomy or thoracotomy, is highly invasive and presents significant risks to the patient. Examples of epicardial defibrillation electrodes are found in U.S. Pat. No. 4,567,900 (Moore), U.S. Pat. No. 4,291,707 (Heilman et al), and U.S. Pat. No. 4,860,769 (Fogarty et al). A pair of differently biased (e.g. oppositely polarized) epicardial electrodes can be employed, as shown in Moore. Alternatively, the Heilman patent discloses an intravenously inserted endocardial electrode arrangement in combination with a patch electrode positioned near the left ventricular apex.
U.S. Pat. No. 4,270,549 (Heilman) describes a technique for inserting and placing defibrillation electrodes, involving intravenous insertion of an endocardial electrode in combination with a patch electrode inserted through a skin incision and through a tunnel created inside the thorax and outside the pleural cavity. Alternatively, U.S. Pat. No. 4,865,037 (Chin et al) discloses a technique for inserting separate electrodes into the intrapericardial space through catheters. An incision is formed in the upper abdominal wall. Then, tissues between the incision and the pericardium are separated, and an incision is then made in the pericardium. A cannula containing a defibrillation electrode is inserted through these incisions, to enable positioning of the electrode in the pericardium. A second cannula containing a second electrode is inserted on the opposite side of the heart, in the same manner.
The above described approaches have enjoyed limited success, yet present risks to patients due to the time and complexity involved. The intravascular approach gives rise to a risk of superior vena cava syndrome, pulmonary embolism, endocardial shock-induced tissue damage, and endocarditis. Left thoracic subcutaneous patches involve discomfort to the patient, and the risk of transcutaneous erosion, subcutaneous infection and fatigue fracture.
Fixation of epicardial electrodes gives rise to difficulties. Active fixation of electrodes, particularly near their free ends, is required to prevent post implantation migration, yet such fixation creates the risk of epicardial lacerations, abrasions and other trauma. Another problem, particularly with patch electrodes, is the current density gradient, i.e. maximum current density regions at the patch periphery. Current density gradients reduce the efficacy of the electrode, in terms of the ratio of useful cardioversion/defibrillation energy as compared to required pulse generator output energy.
Therefore, it is an object of the present invention to provide a tissue stimulation device including two or more differently polarized electrodes, deployable simultaneously using minimally invasive techniques.
Another object is to provide a device for simultaneously deploying multiple, differently biased electrodes within the pericardium.
A further object is to provide a means for deploying an array of resilient electrodes, and for selectively altering the shape of the array following deployment.
Yet another object is to provide an electrode array having a loop configuration and means associated with the loops for reducing current density gradients.